1. Field of the Invention
This invention relates generally to earth anchors, and more particularly, to explosive or belled grouted anchors suitable for anchoring a structure to the ground.
2. Description of the Prior Art
Explosive or Malone anchors, and belled anchors, are known. Such anchors generally comprise an elongated rod or shank having an end configuration which may comprise an integrally formed enlarged end section or an end section formed from a washer and nut screwed onto the rod or shank. Such anchors are generally anchored to the ground by drilling a hole in the ground and setting off an explosive charge at the bottom of the drilled hole. The resultant explosion compresses the earth and forms an enlarged chamber at the bottom of the drilled hole. Alternatively, the chamber may be formed by an expandable drill, such as a belling tool, that is expandable at the bottom end and drills out the chamber. The chamber is then filled with grout through a tube to prevent the grout from being contaminated by foreign matter. The anchor rod is inserted into the drilled hole, and positioned so that the enlarged end section of the anchor rod contacts the bottom of the chamber. The drilled hole is then completely filled with grout. The grout is allowed to harden, and the grout ball thus formed inside the chamber serves to retain the anchor in the ground.
While such anchors are satisfactory for many purposes, in the prior art anchors, no attempt has been made to optimize the shape of the end configuration and its position within the grout ball in order to optimize the transfer of forces from the end configuration through the grout ball and into the soil, while minimizing undesirable tensile stresses within the grout ball. In such prior art anchors, the compressive stress trajectories or lines of force that extend between the surfaces of the end configuration and the soil resisting surface of the grout ball are curved. The compressive stress trajectories are curved because they must intersect the surfaces of the end configuration and the soil resisting surface of the grout ball at 90.degree. angles; and since the end of a stress trajectory intersecting the surface of the end configuration is normally not colinear with the end of the stress trajectory intersecting the surface of the grout ball, the portion of the stress trajectory interconnecting the non-colinear ends must be curved. Such curved trajectories cause shear stresses within the grout ball which result in tensile stresses that cause the grout ball to crack prematurely. Also, when as in the prior art anchors, the end configuration is located in the lower half of the grout ball where there is little or no lateral resistance from the soil, the ball is more prone to splitting due to tensile stresses generated in the region of the end configuration.